The invention relates to gas turbine engines and, more particularly, to a system for controlling the output shaft speed to obtain optimum fuel consumption at a required thrust, especially in low power conditions.
Modern gas turbine engines used in turbopropellers have numerous control features which allow for efficient engine operation over a wide range of operating conditions. Because of the interactions between such diverse features as fuel flow, variable fan pitch, variable fan exhaust nozzle areas, etc., a much more complex control system is required if engine efficiency is to be maintained over a widely varying range of ambient conditions and modes of engine operation.
A free turbine engine provides the opportunity to set propeller shaft speed (Np) independent of gas generator speed. This capability has been used for decades however full advantage has not been taken to optimize fuel consumption particularly at conditions far removed from the normal high power cruise.
Optimizing propulsive efficiency is of particular value to aircraft such as long endurance maritime patrol aircraft where the typical mission comprises dashing to a search area at high speed, patrolling at low speed for as long a time as possible once at the search area and then returning to base. This type of mission requires that the powerplant be optimized at conflicting ends of the operating spectrum.
The traditional approach was to simply accept the higher fuel consumption and to reduce the time spent at the search area. In most cases, the pilots would pull the throttle back, maintain the rotations per minute (RPM) and burn a lot of fuel while patrolling the area. In some aircrafts, it is even possible to shut down one or more engines to cut down on the fuel consumption caused by the patrolling at low speeds.
The traditional turbopropeller powerplant is optimized with propeller speeds chosen for the primary mission (climb and cruise for fuel burning, take-off and maximum continuous for thrust) resulting in poor fuel efficiency at the low power conditions required for loiter. Optimizing for loiter would preclude achievement of acceptable high power performance assuming normal Np.
U.S. Pat. No. 4,258,545 discloses a system for controlling a gas turbine engine using feedback signals, each of which is a function of a plurality of output variables. Control signals are generated and transmitted to actuators which vary engine control variables to provide a desired level of engine performance.
U.S. Pat. No. 4,425,614 discloses a thrust control and thrust specific fuel consumption control by fuel flow rate control.
U.S. Pat. No. 4,488,236 discloses a digital fuel control which controls fuel flow using an estimated optimum minimum speed.
However, none of these systems permit output shaft speed to be controlled. Accordingly, there is a need for an improved control system for an aircraft gas turbine engine.
Accordingly, an object of the present invention is to optimize engine fuel consumption at a given thrust setting by controlling output shaft speed.
Another object of the present invention is to optimize propulsive efficiency.
Another object of the present invention is to add a variable or fixed reduced output shaft speed for use at low power loiter conditions.
Another object of the present invention is to electronically control the output shaft speed on a turbopropeller gas turbine engine in order to optimize the fuel consumption at a required thrust for any given operating conditions of thrust setting, altitude, air speed and ambient temperature.
In accordance with one aspect of the present invention, in a control system for controlling a gas turbine engine, there is provided a method for determining an optimized output shaft speed for a required thrust and setting an appropriate engine power. The method comprises the steps of: providing a required thrust value at a particular flight condition; determining input values for each of a power, output shaft speed, airspeed, and altitude; determining whether the required thrust is a low power condition; if the required thrust is a low power condition, determining a reduced propeller speed value from the input values and the required thrust value; at least one of increasing and decreasing the optimized output shaft speed using the reduced propeller speed value.
In accordance with another aspect of the present invention, there is also provided a system for determining an optimized output shaft speed for a required thrust and setting an appropriate engine power in a gas turbine engine control system. The system comprises: a pilot command providing a required thrust value at a particular flight condition; a reduced power cruise condition determiner determining using the required thrust value, a power, an output shaft speed, an airspeed and an altitude input value whether the required thrust is a low power condition; a reduced output shaft speed determiner for determining a reduced propeller speed value from the input values and the required thrust value when the required thrust is a low power condition; a fuel consumption determiner for at least one of increasing and decreasing the optimized output shaft speed using the reduced propeller speed value.